The present invention relates to centerless grinders.
Centerless grinders are well known machines for grinding elongated cylindrical workpieces such as medical guide wires, rods, pins, golf club shafts, antenna, fishing rods and similar articles. Conventional centerless grinders include a supporting structure on which a grinding wheel and a regulating wheel are mounted with their working surfaces facing each other and slightly separated. The workpiece is positioned between these two wheels (“the working area”). These wheels rotate in the same direction about a substantially horizontal axis at different speeds. The axis of rotation of the regulating wheel is somewhat inclined from the horizontal plane, however, to provide a “tilt angle.” This tilt angle causes the workpiece to move forwardly through the working area as the workpiece is being ground to a particular profile by the grinding wheel. The profile is controlled by moving the regulating wheel toward or away from the grinding wheel as the workpiece passes through the working area.
The grinding wheel typically is approximately 12 inches in diameter and spins at approximately 2500 rpm, and the regulating wheel typically is approximately 6 inches in diameter and spins at approximately 50 rpm. The width of the working surfaces of both the grinding wheel and the regulating wheel typically is approximately 1.5 inches and can range from between 0.25 inches and 4.0 inches. Different surfaces are provided on the grinding wheel and the regulating wheel to provide greater friction between the regulating wheel and the workpiece than that between the grinding wheel and the workpiece. The regulating wheel also may have a plurality of grooves extending circumferentially around its working surface. As a result, the speed and direction of rotation of the workpiece is controlled by the speed and direction of rotation of the regulating wheel, and the grinding wheel moves in a direction of rotation opposite to that of the workpiece to grind the workpiece.
As shown in FIG. 1, in conventional centerless grinders, a work rest blade 109 is positioned in the working area 107 between the grinding wheel 101 and the regulating wheel 103 for supporting the workpiece 105 during the grinding process. The work rest blade may be attached to a holder (not shown) whose surface facing the regulating wheel may be grooved to cooperate with the grooves on the regulating wheel's working surface. A typical work rest blade is extremely thin and has a supporting surface that slopes upwardly at about a 30° angle in the direction from the regulating wheel to the grinding wheel. Highly abrasive particles ground from the workpiece typically collect on this surface. Since the workpiece is rotating in relation to the work rest blade (which is stationary), the workpiece and these particles tend to grind the work rest blade during the grinding process. As a result, the work rest blade must be replaced frequently during typical manufacturing operations resulting in delays in the manufacturing process. Also, for a similar group of workpieces, the work rest blade typically is ground to a specific profile to accommodate the dimensions of the workpiece. Grinding each new work rest blade to such a specific profile further delays the manufacturing process.
Since the work rest blade is positioned between the grinding wheel and the regulating wheel and both of these wheels must simultaneously engage the workpiece, the extent to which the workpiece can be ground is limited by the width of the work rest blade. If the work rest blade is made too narrow, moreover, it will break during the grinding process. In addition, if the work rest blade is ground to a narrow profile to accommodate thin workpieces such as medical guide wires, the extent to which the regulating wheel can be backed away from the grinding wheel also is limited. If the regulating wheel is moved too far away from the grinding wheel, moreover, the workpiece will slip off of the work rest blade resulting in damage to the workpiece and possible damage to the grinding/regulating wheel assembly.
Since the workpiece's forward movement between the grinding wheel and the regulating wheel is controlled by the regulating wheel's speed and tilt angle (among other factors), slight changes in either of these factors can result in errors in the workpiece's desired grinding profile. In order to prevent such errors, systems employing optical sensors, such as those disclosed in U.S. Pat. No. 5,480,342, assigned to Royal Master Grinders, Inc., the assignee of the present application, are used to precisely detect the workpiece's position and to move the regulating wheel in response to this detected position.
A centerless grinder has been produced that operates without a regulating wheel or a work rest blade. In place of the regulating wheel, a spindle having a collet for securing a workpiece 211 (FIGS. 2 and 3) is positioned in front of the working area. Workpiece 211 is passed through the spindle (not shown), and the collet then is tightened around the workpiece. The workpiece then is passed into the working area as a motor rotates the spindle at a particular speed and direction to control the workpiece's speed and direction of rotation adjacent the grinding wheel. As shown in FIGS. 2 and 3, workpiece 211 passes through an elongated bushing 203 in the working area that holds workpiece 211 adjacent the grinding wheel (not shown). Elongated bushing 203 is within a bushing blade 205, and a bushing blade holder 201 supports bushing blade 205. Passageways 207 within bushing blade 203 and bushing blade holder 201 provide coolant to the grinding wheel and workpiece. A slot 209 within bushing blade 205 and elongated bushing 203 enables the grinding wheel to contact workpiece 211. Elongated bushing 203 has an inner diameter slightly greater than the diameter of workpiece 211. Slot 209 is between approximately 0.020 and 0.030 inches in width. To control the workpiece's grinding profile, a first servomotor moves the spindle/collet toward and away from the working area, and a second servomotor moves the elongated bushing toward and away from the grinding wheel.
The grinding wheel for this centerless grinder must be extremely thin to fit into slot 209 in order to contact workpiece 211. The grinding wheel typically is between approximately 1/16 and ⅛ inches in width, and is chamfered at its working surface to fit into this slot. This width is substantially thinner than that of a conventional centerless grinder having a regulating wheel (which, as discussed above, typically is approximately 1.5 inches in width). Because of this thin width, the grinding wheel is fragile and prone to breaking. Also, the grinding process is extremely slow, and the workpiece must be moved through the working area at a rate substantially slower than that for conventional centerless grinders. As a result, the range of useful functions for this centerless grinder is severely limited.
Elongated workpieces typically are fabricated from wire stock shipped on spools. Before the grinding process can begin for a particular set of workpieces, the wire must be dispensed by hand from this spool and cut into a plurality of equal lengths. Each length of wire then must be placed into a feeder for transmitting the wire through the working area. These steps substantially delay the manufacturing process.
Some elongated workpieces require a grinding profile at both ends. After placing the work piece onto the feeder and transmitting the workpiece through the working area, therefore, the workpiece must be removed from the grinder and again placed back on the feeder with its opposite end facing the working area for a second pass through this area. These steps also substantially delay the manufacturing process.